One type of image forming machine employing a printing medium path is a "sheet-feed" printer. In a known "sheet-feed" printer, sheets of paper or other printing media are pulled or picked from an input tray and fed downstream into the print engine components where the desired image is formed on each sheet. This operation is typically accomplished using a series of motor driven rollers that have frictionally adherent surfaces. The surface of the initial or pick roller rotates against the upper surface of the top sheet in the stack to direct the top sheet along a predetermined printing medium path. The sheet is advanced through an infeed zone and is received by a drive roller which supplies the sheet into a processing zone for printing. The sheet is then expelled through an output zone or tray. This cycle is repeated for each sheet to be printed. FIGS. 1a to 1c show a few of the components used in this type of printer.
A problem experienced with printers of this type is that the sheet being fed may become subject to a disturbance somewhere along the printing medium path. The disturbance is typically the result of a sheet having imperfections or becoming skewed in the infeed zone. Usually, the part of the sheet encountering the disturbance will be blocked, whilst the remaining part will continue its progression along the printing medium path by the action of the rollers. Accordingly, the sheet will build up against the blockage until the rollers operating in the direction of the blockage no longer adhere to the fed sheet. Consequently, the sheet will become "jammed" in the printer mechanism as illustrated, for example, in FIGS. 1d and 1e. This effect is known as a sheet or paper jam. Sheet or paper jams can also occur in the other types of printers referred to above which employ printing medium paths.
Although the frequency of paper jams in modern printers has generally decreased, when this problem does occur the result can have a damaging effect on the printer. The printer motor, for example, can over-heat as a result of the increased load on the rollers. Also, in printers using ink, the image forming components may deposit ink within the printer instead of on the printing medium. In order to reduce the damaging effects of paper jams, printers have been developed which can detect certain indicators of a paper jam so that the operations of the printer are ceased as soon as possible.
Once a paper jam has occurred the jammed paper will often end up concertinaed in some form next to the drive roller as illustrated in FIG. 1d. Removal of the paper to clear the jam may then be performed manually by grabbing one of the ends of the paper and pulling. However, this method is undesirable since the paper may rip or unnecessary strain may be placed on the motor as the drive roller is forced around too quickly.
A more suitable and commonly used method of removing a jammed sheet of paper is to manually rotate the drive roller to direct the paper along the printing medium path. This manual rotation may be enabled by a knob located outside the printer housing which is attached to the drive roller. The drive roller may be rotated either in a forwards direction to direct the jammed sheet towards the output zone or in a reverse direction to direct the jammed sheet towards the infeed zone. The choice of direction is arbitrary and both directions are generally attempted, on the basis of trial and error, until successful release of the jammed paper into one of the infeed or output zones.
A drawback with this method is that the manually operable knob located outside the printer housing rotates with the drive roller during normal operation of the printer. Any accidental contact made with the knob will disturb the line feeding of paper by the drive roller. This will result in displacement of the paper during the sensitive printing processing. Consequently, the printing quality will be affected, and particularly so in high resolution printers.
In an alternative system, a shaft is arranged on an axis parallel with the axis of the drive roller. The shaft supports a manually operable knob which is located outside the housing of the printer. When the knob is depressed in an axial direction of the shaft, a gear supported by the shaft engages with a gear attached to the drive roller. A user wishing to clear a paper jam simply pushes the exposed knob towards the body of the printer to engage the drive roller gear train, and turns the knob at the same time to direct the paper away from the drive roller. An advantage of this improved system is that the knob is not rotating while the printer is printing. Thus, accidental contact made with the knob is less likely to disturb the line feeding of paper which occurred in the existing solution.
However, when using the alternative system there is still a considerable risk that the knob will be accidentally pushed in, resulting in disturbance of the line feeding of paper. This is especially so, since in the unengaged position the knob is particularly exposed from the body of the printer.
Furthermore, due to the ergonomics of the alternative system, a user wishing to remove a paper jam will be required to perform repeated "pushes" and "turns" of the knob. This problem has been addressed in known systems by increasing the gear ratio of the gear train so that each turn of the knob will cause increased rotation of the drive roller. However, this fix solution is not ideal since fast turning of the knob will generate undue stress to the gear train and drive motor. Additionally, the frictional resistance of the system will increase so that turning of the knob will be harder for a user.
In certain printers such as the DeskJet 1100C, available from the Hewlett-Packard Company, USA, the pick roller is selectively coupled to the drive roller via a gear and clutch arrangement. A short reverse rotation of the drive roller causes the pick roller to engage with the drive roller for one rotation of the pick roller in order to pick one sheet of paper from the paper stack. Because during normal operation the drive roller only performs forward rotations, the short reverse rotation provides a convenient initiation of the sheet picking operation. However, when removing paper jams using the aforementioned manual systems, the drive roller will typically be reversed and further sheets will be picked from the sheet stack. This is undesirable since the newly fed sheets may compound the jamming problem by becoming jammed themselves. Moreover, these extra sheets will be wasted.